1. Field of the Invention
The present invention relates to a self-oscillation type switching power supply apparatus.
2. Description of the Related Art
As a self-oscillation type switching power supply apparatus, ringing choke converters have been conventionally used in most cases. FIG. 3 is a circuit diagram showing the structure of a conventional ringing choke converter. As seen in FIG. 3, an AC power supply AC is connected to the input terminal of a rectifying diode bridge DB1 through a fuse F and a line filter LPF. To the output terminal of the diode bridge DB1, a smoothing capacitor C1 is connected. A rectifying smoothing circuit formed of a rectifying diode D3 and a smoothing capacitor C5 in series with each other are connected across the opposite ends of a secondary winding N2 of a transformer T. At the output of the rectifying smoothing circuit, a resistor voltage dividing circuit formed of resistors R9, R10 and a voltage detecting circuit made up of a shunt regulator SR, a light emitting diode PD of a photocoupler, and a resistor R8 are provided.
A switching transistor Q1 is connected in series with a primary winding N1 of the transformer T. To a feedback winding N.sub.B of the transformer T, a controlling circuit containing a phototransistor PT as the photodetector of the photocoupler is connected. A starting-up resistor R1 is connected between the gate of the switching transistor Q1 and the input power-supply voltage supplying-section.
The operation of the circuit shown in FIG. 3 will be now described. At start up when the power supply is put to work, voltage is applied to the gate of the switching transistor Q1 through the starting resistor R1, so that the switching transistor Q1 is turned on. Thereby, an input power supply voltage is applied to the primary winding N1 of the transformer T, so that a voltage is produced in the feedback winding N.sub.B in the same direction as that in the primary winding N1. The voltage signal is provided as a positive feedback signal to the gate of the switching transistor Q1 through a resistor R2 and a capacitor C2. The voltage produced in the feedback winding N.sub.B also causes a charging current to flow to a capacitor C3 through a diode D1, resistors R3, R5, and the phototransistor PT of the photocoupler. The capacitor C3 and a resistor R4 constitute a time constant circuit, and when the charging voltage of the capacitor C3 exceeds the base--emitter forward voltage of the controlling transistor Q2, the controlling transistor Q2 is turned on. Thereby, the gate--source voltage of the controlling transistor Q1 becomes substantially zero, so that the controlling transistor Q1 is forced to turn off. At this time, a voltage is produced in the secondary winding of the transformer T in the forward direction with respect to the rectifying diode D3, and thereby, energy stored in the transformer T while Q1 is on is released through the secondary winding N2. The diode D2, the resistors R6, R7, and the capacitor C4 form a discharging time constant setting circuit for the capacitor C3. With this circuit, the capacitor C3 is reverse-charged (discharged) with the flyback voltage from the feedback winding N.sub.B.
When the voltage of the capacitor C3 goes lower than the base--emitter forward voltage of the controlling transistor Q2, the controlling transistor Q2 is turned off. The energy stored in the transformer T is released into the secondary, and the current in the rectifying diode D3 becomes zero. Then, the respective winding voltages of the transformer T become zero. The circuit is returned to its initial state, and then, the switching transistor Q1 is turned on. Succeedingly, the above-described operation is repeated.
The output voltage on the load side is detected as a voltage divided by the resistors R9, R10. The divided voltage is applied as a detection voltage and a control voltage for the shunt regulator SR. The shunt regulator SR regulates the conducting quantity of electricity for the light emitting diode PD of the photocoupler in dependence on the detection voltage. By the regulation, the quantity of light received by the phototransistor PT, as the photodetector of the photocoupler, is changed, causing the impedance of the phototransistor PT to change. As a result, the charging time constant of the capacitor C3 is changed. As the output voltage is lower, so the charging time constant is higher. That is, as the output voltage is lower, the period from the time when the switching transistor Q1 is turned on until it is forced to be turned off by the controlling transistor Q2, namely, the on-state time period of the switching transistor Q1 is longer. This acts so that the output voltage is increased. Thus, the constant voltage control for keeping the output voltage constant is achieved.
In the conventional self-oscillation type switching power supply apparatus as shown in FIG. 3, in the event that the input power-supply voltage is reduced to be lower than the rated voltage, for example, the AC power supply AC is interrupted, the voltage applied to the primary winding N1 of the transformer T when the switching transistor Q1 is on is reduced. In proportion to the reduction of the voltage, the voltage produced in the feedback winding N.sub.B is reduced. Accordingly, even if the load is constant, the rising of the charging voltage of the capacitor C3 lags, so that the period from the time when the switching transistor Q1 is turned on to the time when the controlling transistor Q2 is turned on, namely the on-state time-period of transistor Q1 becomes longer. As a result, as the input power-supply voltage is reduced, the output voltage supplied to the load is temporarily raised (spring up). Thereafter, the output voltage is gradually reduced (hereinafter, the spring-up of the output voltage occurring when the input power-supply voltage is reduced is referred to as overshoot). Especially, when the load is light, the rising of the output voltage, caused by the increase of the on-state time-period of the switching transistor Q1, is directly reflected so that a significant overshoot occurs. Such an overshoot may cause an error operation or fault of the connected powered circuit, depending on the type of the load.